The influence of pressure on the formation of macroscopic & microscopic gas hydrate structures and concentration efficiency in coffee solutions

Abstract

Gas hydrates, solid structures made of water and gas, can bind high amounts of water. Therefore, current research aimed at concentrating food extracts using gas hydrates for a novel low-temperature concentration technology. Well reproducible gas hydrate formation was observed in 13 wt% coffee solutions in a high-pressure reactor at target pressures of 3.6 and 4.1 MPa, and a cooling temperature of 274.15 K. Different hydrate structures with varying hydrate-to-coffee ratios along with concentrated coffee were obtained from this process and analysed. The coffee total solids content (TS) was ranging from 3 wt% in the hydrate phase to 21 wt% in the coffee phase resulting in a concentration degree of around 62%. Higher concentrations were obtained for higher target pressures. The hydrate phase was made of up to 18 wt% gas, decreasing with decreasing target pressure. A dense layer of hydrate was observed on the (cooled) vessel wall, the thickness of which increased with increasing target pressure. This formation of compact, dense hydrate structures allowed for a simple separation of gas hydrates from the remaining coffee concentrate. This study also includes the first ever cryo-scanned electron microscopy analysis (SEM) of gas hydrates generated in food extracts. In summary, the principle of concentrating coffee solutions using gas hydrate technology has been proven. With increasing target pressures, the thickness of the compact hydrate layer at the vessel wall increased, and better capture of CO₂ in the hydrate as well as a higher concentration of the food extract were observed.